Due to increased fuel costs and more stringent engine emission standards, efforts towards reducing fuel consumption while retaining the existing levels of vehicle performance have been intensified in the recent past. One common approach towards achieving this objective has been to increase the drive axle ratio, a procedure which allows the engine to be run at lower speeds and which results in lower fuel consumption. One inherent drawback with the approach of increasing the drive axle ratio while using an engine of the same general layout is that the available torque in the lower engine speed range is often insufficient. As a result, the vehicle is often driven in lower gears, which will nullify the fuel economy advantage that is being sought.
In an effort to increase engine efficiency, it has been proposed in the aforementioned German DE-OS No. 23 39 356 to provide relatively long intake conduits between the plenum and the intake ports of the cylinders. In this prior-art engine, the plenum is disposed on top of the cylinder head cover. The individual intake conduits extend along arcuate paths from the plenum to the lateral intake ports of the cylinders and are therefore relatively long. A shortcoming of this arrangement is that the intake conduits are of arc-shaped configuration and that, as a result, the flow losses are relatively great. Furthermore, since the plenum in this prior-art intake manifold system is mounted on top of the cylinder head cover, the freedom of arranging the individual intake conduits is severely restricted, so that tuning of the gas flow oscillations is feasible only to a very limited degree.
In order to keep the flow losses in the individual intake conduits relatively low, it has been common practice to employ intake conduits that extend in a straight line, as disclosed in German patent DE-PS No. 23 45 930. In an arrangement of this type, the individual intake conduits are used to support the plenum. However, because of space and stability considerations, such a design precludes the use of longer intake pipes, a feature which is very desirable if engine efficiency is to be improved.